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A window substrate for protecting an electronic display device includes a
first glass substrate having a first thickness, a second glass substrate
having a second thickness, and an interlayer disposed between the first
glass substrate and the second glass substrate, the interlayer having a
third thickness, wherein the sum of the first thickness, the second
thickness, and the third thicknesses is equal to or less than about 190
.mu.m. A display device including a display panel to display an image and
a window substrate also is disclosed.

1. A window substrate for protecting an electronic display device, the
window substrate comprising: a first glass substrate having a first
thickness; a second glass substrate having a second thickness; and an
interlayer disposed between the first glass substrate and the second
glass substrate, the interlayer having a third thickness, wherein the sum
of the first thickness, the second thickness, and the third thickness is
about 190 .mu.m or less.

2. The window substrate of claim 1, wherein each of the first thickness
and the second thickness is about 50 .mu.m or less.

3. The window substrate of claim 2, wherein the electronic display device
is a flexible display device and when the window substrate is bent such
that the first glass substrate is disposed toward the outside of the
flexible display device and the second glass substrate is disposed toward
the inside of the flexible display device, the first thickness is smaller
than the second thickness.

4. The window substrate of claim 3, wherein the first thickness is about
40 .mu.m or less, and the second thickness is about 50 .mu.m or less.

5. The window substrate of claim 1, wherein the third thickness is about
100 .mu.m or less.

6. The window substrate of claim 5, wherein the third thickness is about
10 .mu.m to about 30 .mu.m.

8. The window substrate of claim 1, wherein the window substrate has an
impact resistance such that the window substrate is not damaged when a
pen of 5.8 g is vertically dropped from a first height onto one surface
of the window substrate, and wherein the first height is about 4 cm or
more.

9. The window substrate of claim 1, wherein at least one of the first and
second glass substrates includes a first region extending from one
surface thereof to a first depth at which an ion exchange process occurs,
and having a first compressive stress.

10. The window substrate of claim 9, wherein the first depth is about
1.mu.m or more, and the first compressive stress is about 600 MPa to
about 1200 MPa.

11. The window substrate of claim 1, wherein the window substrate has a
light transmittance of about 90% or more.

12. The window substrate of claim 1, wherein a yellow index variation of
the window substrate is about 2.0 or less when the window substrate is
exposed to UVB light having a wavelength of 280 nm to 360 nm for 72
hours.

13. The window substrate of claim 1, wherein an absorption load when an
iron ball of 5.5 g is freely dropped on one surface of the window
substrate is 5% or more of an impact load of the iron ball.

14. The window substrate of claim 1, further comprising a cover layer
disposed on the first glass substrate or the second glass substrate.

15. The window substrate of claim 14, wherein the cover layer comprises
at least one of an anti-reflection layer, an anti-stain layer, or an
anti-fingerprint layer.

16. The window substrate of claim 1, wherein at least a portion of the
window substrate has flexibility.

17. A display device comprising: a display panel configured to display an
image on a front surface thereof; and a window substrate disposed on the
front surface of the display panel, wherein the window substrate
comprises: a first glass substrate having a first thickness; a second
glass substrate having a second thickness; and an interlayer disposed
between the first glass substrate and the second glass substrate, the
interlayer having a third thickness, wherein the sum of the first
thickness, the second thickness, and the third thickness is about 190
.mu.m or less.

18. The display device of claim 17, wherein each of the first thickness
and the second thickness is about 50 .mu.m or less.

19. The display device of claim 18, wherein the window substrate is
bendable into a position such that when the first glass substrate is
disposed toward the outside of the display device and the second glass
substrate is disposed toward the inside of the display device, the first
thickness is smaller than the second thickness.

20. The display device of claim 19, wherein one surface of the display
panel is in contact with one surface of the second glass substrate.

Description

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims priority from and the benefit of Korean
Patent Application No. 10-2016-0147526, filed on Nov. 7, 2016, which is
hereby incorporated by reference for all purposes as if fully set forth
herein.

BACKGROUND

Field

[0002] The invention generally relates to a window substrate, and more
particularly, to a flexible window substrate and a display device having
the same.

[0004] Since flexible display devices have bending and folding
characteristics, the flexible display devices can be folded or rolled.
Accordingly, the flexible display devices having large screens can be
conveniently carried. The flexible display devices can be applied in
various fields including not only mobile devices such as mobile phones,
portable multimedia players (PMPs), navigations, ultra mobile PCs
(UMPCs), electronic books, and electronic newspapers, but also TVs,
monitors, and the like.

[0005] Defects may occur in the flexible display devices when an external
shock force is applied, including defects affecting display quality or
resolution. In this case, to avoid deterioration of display quality,
flexible display devices should be resistant to external shock. With the
advent of the flexible display devices, the materials used to protect
them from external shocks have become thinner.

[0006] The above information disclosed in this Background section is only
for enhancement of understanding of the background of the inventive
concepts, and, therefore, it may contain information that does not
constitute prior art.

SUMMARY

[0007] Window substrates constructed according to the principles of the
invention are capable of protecting the display devices from an external
shock and/or preventing the display devices damaged even when they are
folded multiple times while maintaining their flexible characteristics.
For example, Applicant discovered that a window substrate having at least
two glass substrates, with each substrate having a predetermined
thickness to provide a compressive stress profile, and an interlayer
disposed between the glass substrates can provide excellent resistance to
external shock while also preventing damage caused by folding the display
multiple times.

[0008] Exemplary embodiments also provide a display device having a window
substrate constructed according to the principles of the invention.

[0009] Additional aspects will be set forth in the detailed description
which follows, and, in part, will be apparent from the disclosure, or may
be learned by practice of the inventive concepts.

[0010] According to one aspect of the invention, a window substrate for
protecting an electronic display device includes: a first glass substrate
having a first thickness; a second glass substrate having a second
thickness; and an interlayer disposed between the first glass substrate
and the second glass substrate, the interlayer having a third thickness,
wherein the sum of the first thickness, the second thickness, and the
third thicknesses is about 190 .mu.m or less.

[0011] Each of the first thickness and the second thickness may be about
50 .mu.m or less.

[0012] The electronic display device may be a flexible display and when
the window substrate is bent such that the first glass substrate is
disposed toward the outside of the flexible display device and the second
glass substrate is disposed toward the inside of the flexible display
device, the first thickness may be smaller than the second thickness. The
first thickness may be about 40 .mu.m or less, and the second thickness
may be about 50 .mu.m or less.

[0013] The third thickness may be about 100 .mu.m or less. Alternatively,
the third thickness may be about 10 .mu.m to about 30 .mu.m.

[0014] The interlayer may include an optically clear adhesive.

[0015] The window substrate may have an impact resistance such that the
window substrate is not damaged when a pen of 5.8 g is vertically dropped
from a first height onto one surface of the window substrate, and wherein
the first height may be about 4 cm or more.

[0016] At least one of the first and second glass substrates may include a
first region extending from one surface thereof to a first depth at which
an ion exchange process occurs, and having a first compressive stress.
The first depth may be about 1.mu.m or more, and the first compressive
stress may be about 600 MPa to about 1200 MPa.

[0017] The window substrate may have a light transmittance of about 90% or
more.

[0018] A yellow index variation of the window substrate may be about 2.0
or less when the window substrate is exposed to UVB light having a
wavelength of 280 nm to 360 nm for 72 hours.

[0019] An absorption load when an iron ball of 5.5 g is freely dropped on
one surface of the window substrate may be 5% or more of an impact load
of the iron ball.

[0020] The window substrate may further include a cover layer disposed on
the first glass substrate or the second glass substrate.

[0021] The cover layer may include at least one of an anti-reflection
layer, an anti-stain layer, or an anti-fingerprint layer.

[0022] At least a portion of the window substrate may have flexibility.

[0023] The window substrate may be used in a display device together with
a display panel. More specifically, and according to another aspect of
the invention, a display device includes: a display panel configured to
display an image on a front surface thereof, and the window substrate is
disposed on the front surface of the display panel. The window substrate
may include a first glass substrate having a first thickness; a second
glass substrate having a second thickness; and an interlayer disposed
between the first glass substrate and the second glass substrate, the
interlayer having a third thickness, wherein the sum of the first
thickness, the second thickness, and the third thickness is about 190
.mu.m or less.

[0024] When the window substrate is bendable into a position such that the
first glass substrate is disposed toward the outside of the display
device and the second glass substrate is disposed toward the inside of
the display device, the first thickness may be smaller than the second
thickness. One surface of the display panel may be in contact with one
surface of the second substrate.

[0025] The foregoing general description and the following detailed
description are exemplary and explanatory and are intended to provide
further explanation of the claimed subject matter.

BRIEF DESCRIPTION OF THE DRAWINGS

[0026] The accompanying drawings, which are included to provide a further
understanding of the inventive concepts, and are incorporated in and
constitute a part of this specification, illustrate exemplary embodiments
of the inventive concepts, and, together with the description, serve to
explain principles of the inventive concepts.

[0027] In the drawing figures, dimensions may be exaggerated for clarity
of illustration. It will be understood that when an element is referred
to as being "between" two elements, it can be the only element between
the two elements, or one or more intervening elements may also be
present. Like reference numerals refer to like elements throughout.

[0028] FIG. 1A is a perspective view illustrating a first embodiment of a
window substrate constructed according to the principles of the
invention.

[0029] FIG. 1B is a cross-sectional view of the window substrate of FIG.
1A in an unfolded position taken along line I-I' of FIG. 1A.

[0030] FIG. 1C is a cross-sectional view of the window substrate of FIG.
1A in a folded position taken along line I-I' of FIG. 1A.

[0031] FIG. 2A is a schematic, cross-sectional view of a first or second
glass substrate in the window substrate according to one or more
exemplary embodiments.

[0032] FIG. 2B is a graph illustrating stress versus distance from a
surface of the glass substrate of FIG. 2A.

[0033] FIG. 3 is a cross sectional view of a second embodiment of a window
substrate constructed according to the principles of the invention.

[0034] FIG. 4 is a cross sectional view of a third embodiment of a window
substrate constructed according to the principles of the invention.

[0035] FIG. 5 is a perspective view of a first embodiment of a display
device constructed according to the principles of the invention having a
window substrate according to FIG. 1A.

[0036] FIG. 6A is a cross-sectional view of the display device of FIG. 5
in an unfolded position taken along line II-II' of FIG. 5.

[0037] FIG. 6B is a cross-sectional view of the display device of FIG. 5
in a folded position taken along line II-II' of FIG. 5.

[0038] FIG. 7 is a block diagram of the major electronic components of a
display panel constructed according to the principles of the invention.

[0039] FIG. 8 is a circuit diagram of one of the pixels in FIG. 7.

[0040] FIG. 9 is a perspective view illustrating a second embodiment of a
display device constructed according to the principles of the invention
having a window substrate according to FIG. 1A foldable about a single
folding line.

[0041] FIG. 10 is a plan view illustrating a display device according to
one or more exemplary embodiments schematically illustrating only two
rigid areas, a flexible area, and a folding line.

[0042] FIG. 11A is a perspective view illustrating a third embodiment of a
display device constructed according to the principles of the invention
having a window substrate according to FIG. 1A foldable about two folding
lines.

[0043] FIG. 11B is a perspective view illustrating the display device of
FIG. 11A in a folded position.

[0044] FIG. 12A is a perspective view illustrating a fourth embodiment of
a display device constructed according to the principles of the invention
having a window substrate according to FIG. 1A foldable about a single,
offset folding line.

[0045] FIG. 12B is a perspective view illustrating the display device of
FIG. 12A in a rolled position.

[0046] FIG. 13 is a graph illustrating impact resistances of an existing
window substrate and a window substrate constructed according to the
principles of the invention.

[0047] FIG. 14 is a graph illustrating impact loads of in window
substrates constructed according to the principles of the invention when
the thicknesses of first and second glass substrates are constant but the
thicknesses of interlayers vary.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

[0048] In the following description, for the purposes of explanation,
numerous specific details are set forth in order to provide a thorough
understanding of various exemplary embodiments. It is apparent, however,
that various exemplary embodiments may be practiced without these
specific details or with one or more equivalent arrangements. In other
instances, well-known structures and devices are shown in block diagram
form in order to avoid unnecessarily obscuring various exemplary
embodiments.

[0049] In the accompanying figures, the size and relative sizes of layers,
films, panels, regions, etc., may be exaggerated for clarity and
descriptive purposes. Also, like reference numerals denote like elements.

[0050] When an element or layer is referred to as being "on," "connected
to," or "coupled to" another element or layer, it may be directly on,
connected to, or coupled to the other element or layer or intervening
elements or layers may be present. When, however, an element or layer is
referred to as being "directly on," "directly connected to," or "directly
coupled to" another element or layer, there are no intervening elements
or layers present. For the purposes of this disclosure, "at least one of
X, Y, and Z" and "at least one selected from the group consisting of X,
Y, and Z" may be construed as X only, Y only, Z only, or any combination
of two or more of X, Y, and Z, such as, for instance, XYZ, XYY, YZ, and
ZZ. Like numbers refer to like elements throughout. As used herein, the
term "and/or" includes any and all combinations of one or more of the
associated listed items.

[0051] Although the terms first, second, etc. may be used herein to
describe various elements, components, regions, layers, and/or sections,
these elements, components, regions, layers, and/or sections should not
be limited by these terms. These terms are used to distinguish one
element, component, region, layer, and/or section from another element,
component, region, layer, and/or section. Thus, a first element,
component, region, layer, and/or section discussed below could be termed
a second element, component, region, layer, and/or section without
departing from the teachings of the present disclosure.

[0052] Spatially relative terms, such as "beneath," "below," "lower,"
"above," "upper," and the like, may be used herein for descriptive
purposes, and, thereby, to describe one element or feature's relationship
to another element(s) or feature(s) as illustrated in the drawings.
Spatially relative terms are intended to encompass different orientations
of an apparatus in use, operation, and/or manufacture in addition to the
orientation depicted in the drawings. For example, if the apparatus in
the drawings is turned over, elements described as "below" or "beneath"
other elements or features would then be oriented "above" the other
elements or features. Thus, the exemplary term "below" can encompass both
an orientation of above and below. Furthermore, the apparatus may be
otherwise oriented (e.g., rotated 90 degrees or at other orientations),
and, as such, the spatially relative descriptors used herein interpreted
accordingly.

[0053] The terminology used herein is for the purpose of describing
particular embodiments and is not intended to be limiting. As used
herein, the singular forms, "a," "an," and "the" are intended to include
the plural forms as well, unless the context clearly indicates otherwise.
Moreover, the terms "comprises," "comprising," "includes," and/or
"including," when used in this specification, specify the presence of
stated features, integers, steps, operations, elements, components,
and/or groups thereof, but do not preclude the presence or addition of
one or more other features, integers, steps, operations, elements,
components, and/or groups thereof.

[0054] Various exemplary embodiments are described herein with reference
to sectional illustrations that are schematic illustrations of idealized
exemplary embodiments and/or intermediate structures. As such, variations
from the shapes of the illustrations as a result, for example, of
manufacturing techniques and/or tolerances, are to be expected. Thus,
exemplary embodiments disclosed herein should not be construed as limited
to the particular illustrated shapes of regions, but are to include
deviations in shapes that result from, for instance, manufacturing. For
example, an implanted region illustrated as a rectangle will, typically,
have rounded or curved features and/or a gradient of implant
concentration at its edges rather than a binary change from implanted to
non-implanted region. Likewise, a buried region formed by implantation
may result in some implantation in the region between the buried region
and the surface through which the implantation takes place. Thus, the
regions illustrated in the drawings are schematic in nature and their
shapes are not intended to illustrate the actual shape of a region of a
device and are not intended to be limiting.

[0055] Unless otherwise defined, all terms (including technical and
scientific terms) used herein have the same meaning as commonly
understood by one of ordinary skill in the art to which this disclosure
is a part. Terms, such as those defined in commonly used dictionaries,
should be interpreted as having a meaning that is consistent with their
meaning in the context of the relevant art and will not be interpreted in
an idealized or overly formal sense, unless expressly so defined herein.

[0056] One or more exemplary embodiments relate to a window substrate
including a glass substrate. The window substrate can be employed in a
display device. Particularly, the window substrate can be used as a
window panel located on a front surface of a display panel. Accordingly,
in one or more exemplary embodiments, the window panel is referred to as
a window substrate. However, the use of the window substrate constructed
according to the principles of the invention is not limited thereto, and
the window substrate may be used in any other application requiring a
transparent insulating substrate. For example, the window substrate may
be used as a base substrate on which elements are mounted in the display
device or a counter substrate opposite to the base substrate. In
addition, the window substrate may be used as a substrate of a touch
screen panel disposed on a display substrate.

[0057] FIG. 1A is a perspective view illustrating a first embodiment of a
window substrate constructed according to the principles of the
invention. FIG. 1B is a cross-sectional view of the window substrate of
FIG. 1A in an unfolded position taken along line I-I' of FIG. 1A. FIG. 1C
is a cross-sectional view of the window substrate of FIG. 1A in a folded
position taken along line I-I' of FIG. 1A

[0058] Referring to FIGS. 1A to 1C, the window substrate WD according to
the illustrated embodiment is shown as having a rectangular, plate shape,
but any other shape known in the art may be used as well.

[0059] In particular, for convenience of description, it has been assumed
that the planar shape of the window substrate WD when viewed in plan is a
rectangular shape having a pair of long sides and a pair of short sides.
In addition, it has been illustrated that the long sides extend
longitudinally in a first direction D1, and the short sides extend in a
second direction D2 orthogonal to the first direction D1, and the third
direction D3 is perpendicular to the both the first and second directions
D1 and D2. However, the shape of the window substrate WD is not limited
thereto, but the window substrate WD may have various shapes. For
example, the window substrate WD may be provided in various shapes such
as a closed-shape polygon including linear sides, a circle, an ellipse,
etc., including curved sides, and a semicircle, a semi-ellipse, etc.,
including linear and curved sides. In one embodiment, when the window
substrate WD has linear sides, at least one portion of corners of each of
the shapes may be formed in a curve. For example, when the window
substrate WD has a rectangular shape, a portion at which adjacent linear
sides meet each other may be replaced with a curve having a predetermined
curvature. That is, a vertex portion of the rectangular shape may be
formed with a curved side having both adjacent ends respectively
connected to two adjacent linear sides, the curved side having a
predetermined curvature. The curvature may be differently defined
depending on position. For example, the curvature may be changed
depending on the position at which the curve is started, a length of the
curve, etc.

[0060] The window substrate WD includes a first glass substrate GL1, a
second glass substrate GL2, and an interlayer ML between the first glass
substrate GL1 and the second glass substrate GL2.

[0061] Each of the first glass substrate GL1 and the second glass
substrate GL2 is provided in a plate shape having opposed surfaces. Each
of the first glass substrate GL1 and the second glass substrate GL2 may
be made of a glass material including silicate. In one or more exemplary
embodiments, various materials may be further added to the glass material
to provide for excellent durability, surface smoothness, and
transparency. For example, the glass substrate may be made of a material
including aluminosilicate, borosilicate, boroaluminosilicate, etc. In one
or more exemplary embodiments, the glass substrate may further include
alkali metal or alkali earth metal and oxide thereof.

[0062] In one or more exemplary embodiments, the material of the first and
second glass substrates GL1 and GL2 is not limited to the above-described
compositions, and the first and second glass substrates GL1 and GL2 may
be modified using various other materials and various ratios thereof
known in the art.

[0063] The interlayer ML allows the first glass substrate GL1 and the
second glass substrate GL2 to be attached to each other. Also, the
interlayer ML distributes stresses generated in the first glass substrate
GL1 and the second glass substrate GL2. In other words, when an impact is
applied to a portion of the first glass substrate GL1 and/or the second
glass substrate GL2, the interlayer ML offsets a tensile stress generated
in the glass substrate due to the impact to prevent the glass substrate
from being broken, and absorbs impact energy generated when the glass
substrate is broken to prevent minute glass fragments from being
scattered. To this end, the interlayer ML may have an elastic material to
absorb impact energy, and have a flexible material that is curvable or
foldable.

[0064] The interlayer ML may be made of an optically clear glue or
adhesive. The material constituting the interlayer may include an acrylic
polymer, an ethylene vinyl acetate polymer, a nitrile polymer, a silicone
rubber, a butyl rubber, a styrene block copolymer, a vinyl ether polymer,
a urethane polymer, an epoxy polymer, and the like. For example, the
interlayer ML may be made of a urethane polymer, or may be made of the
urethane polymer to which a rubber or acrylic polymer is added. However,
the material constituting the interlayer ML is not limited thereto, and
may include various adhesive materials known in the art.

[0065] In one or more exemplary embodiments, the window substrate may have
flexibility and accordingly can be curved, folded, or rolled. In one or
more exemplary embodiments, the window substrate may be folded or rolled
in the third direction D3 or the direction opposite to the third
direction D3. That is, the window substrate may be folded in a direction
in which a portion of the outer surface of the second glass substrate GL2
faces the rest of the outer surface of the second glass substrate GL2.
The term "folded" or "bended" does not mean a fixed shape but means a
shape deformable into another shape from the original shape, and includes
a folded, bent, curved, or rolled shape, such as a roll along one or more
specific lines.

[0066] In one or more exemplary embodiments, the first glass substrate
GL1, the second glass substrate GL2, and the interlayer ML may have a
total thickness of about 190 .mu.m or less. That is, if a distance
between the opposed surfaces of the first glass substrate GL1 is a first
thickness t1, a distance between the opposed surfaces of the second glass
substrate GL2 is a second thickness t2, and a thickness of the interlayer
ML between the first glass substrate GL1 and the second glass substrate
GL2 is a third thickness t3, the sum of the first to third thicknesses t1
to t3 may be about 190 .mu.m or less. Applicants have discovered that the
total maximum thickness of about 190 .mu.m is particularly advantageous.
When the window substrate has a thickness of over 190 .mu.m, the
repulsive force of the window substrate against deformation is
excessively increased, and therefore, the bending of the window substrate
is difficult.

[0067] In one or more exemplary embodiments, the first glass substrate
GL1, the second glass substrate GL2, and the interlayer ML may have a
total thickness of about 50 .mu.m or more. That is, the sum of the first
to third thicknesses t1 to t3 may be about 50 .mu.m or more. When the
thickness of the window substrate WD is less than 50 .mu.m, the rigidity
of the window substrate WD is low, and therefore, the window substrate WD
may be damaged in processing. However, it is preferable to make the
thickness of the window substrate WD as thin as possible, and therefore,
the window substrate WD may be fabricated to have a thickness of less
than about 50 .mu.m within a limit that satisfies rigidity and processing
ability. That the window substrate WD has been damaged means a state in
which the window substrate WD cannot be used for an intended purpose,
such as a state in which the window substrate WD is broken, a state in
which a flaw or crack is generated in the window substrate WD, a state in
which the flaw or crack is propagated, or a state in which the window
substrate WD is ruptured.

[0068] In detail, the first thickness t1 of the first glass substrate GL1
may be about 10 .mu.m to about 50 .mu.m, and the second thickness t2 of
the second glass substrate GL2 may be about 10 .mu.m to about 50 .mu.m.
In addition, the third thickness t3 of the interlayer ML may be about 10
.mu.m to about 100 .mu.m. In another embodiment of the invention, the
first thickness t1 may be about 30 .mu.m to about 50 .mu.m, and the
second thickness t2 may be about 30 .mu.m to about 50 .mu.m. In addition,
the third thickness t3 may be about 10 .mu.m to about 30 .mu.m.

[0069] Consequently, the window substrate WD constructed according to the
principles of the invention is bendable with a relatively small radius of
curvature R. Hence, the window substrate WD may have a radius of
curvature R of about 1 mm to about 10 mm or about 1 mm to about 5 mm.

[0070] In one or more exemplary embodiments, the thicknesses of the first
glass substrate GL1 and the second glass substrate GL2 may be equal to
each other, or may be different from each other. Particularly, when the
window substrate WD is folded such that, based on any one surface of the
window substrate WD, a portion of the surface and the rest of the surface
face each other, the thicknesses of the first glass substrate GL1 and the
second glass substrate GL2 may be different so as to minimize stresses
applied to the first and second glass substrates GL1 and GL2.

[0071] For example, when a reference line about which the window substrate
WD is folded is denoted as a folding line FL, the folding line FL may be
located at a surface of the second glass substrate GL2 of the window
substrate WD. The window substrate WD may be folded, about the folding
line FL, such that a portion of the surface of the second glass substrate
GL2 and the rest of the surface of the second glass substrate GL2 face
each other. In this case, the inside of the second glass substrate GL2 is
disposed at the outside of the first glass substrate GL1, one surface of
the first glass substrate GL1 becomes an outer circumferential surface,
and one surface of the second glass substrate GL2 becomes an inner
circumferential surface. When the window substrate WD is bent, the first
thickness t1 of the first glass substrate GL1 at the outside of the
window substrate WD may have a smaller value than the second thickness t2
of the second glass substrate GL2 disposed at the inside. In an
embodiment, the first thickness t1 may be about 20 .mu.m to about 40
.mu.m, and the second thickness t2 may be 30 .mu.m to about 50 .mu.m.

[0072] As described above, the window substrate WD includes two glass
substrates having a small thickness of 50 .mu.m or less. Hence, when an
object having a narrow cross sectional area, such as a pen, collides with
the window substrate, bending breakage may occur as the glass substrate
is broken by stress applied to the glass substrate. In order to reduce
the bending breakage, the impact resistance of the glass substrate must
be improved. To this end, the glass substrate constructed according to
the principles of the invention may be a glass chemically reinforced by
an ion exchange process so as to improve the impact resistance thereof.
In one or more exemplary embodiments, the term "ion exchange process"
means that the glass exchanges a positive ion of the same atomic value
with a positive ion located on or near the surface of the glass. For
example, the ion exchange process may mean that a positive ion (e.g., a
positive ion of an alkali metal such as Na.sup.+ or Li.sup.+) inside the
glass is exchanged with another positive ion from the outside. The ion
exchange process may be used to increase the compressive stress profile
of the glass to a specific depth extending from both surfaces of the
glass. When such a compressive stress profile is provided in the glass
substrate, a high strength is provided against bending of the glass
substrate as long as the flaw exists within a region defined by a
reference line where the compressive stress is above 0 in a compressive
stress graph. Here, the defined region may be a region between the
surface of the glass and a specific depth from the surface. Accordingly,
chemical reinforcement of both surfaces of the glass may be achieved by
the ion exchange process.

[0073] FIG. 2A is schematic, cross-sectional view of a first or second
glass substrate GL1 or GL2 in the window substrate according to one or
more exemplary embodiments. FIG. 2B is a graph illustrating stress versus
distance from a surface of the glass substrate of FIG. 2A. The first
glass substrate GL1 and the second glass substrate GL2 have the
substantially same shape except their thicknesses. Therefore,
hereinafter, the first glass substrate GL1 and the second glass substrate
GL2 are commonly called as a glass substrate.

[0074] Referring to FIGS. 2A and 2B, the glass substrate includes a first
region RG1 extending to a first depth DOL1 from one surface (hereinafter,
referred to as a first surface S1) thereof, a second region RG2 extending
to a second depth DOL2 from the other surface (hereinafter, referred to
as a second surface S2) thereof, and a third region RG3 located between
the first region RG1 and the second region RG2.

[0075] In the first region RG1, the first depth DOL1 is a depth at which a
positive ion inside the glass substrate is exchanged with a positive ion
from the outside. Accordingly, the first region RG1 has a first thickness
T1. A first compressive stress CS1 is applied to the first region RG1 by
the ion exchange. The first compressive stress CS1 decreases from the
first surface S1 to the first depth DOL1 according to a predetermined
function, and becomes 0 at the first depth DOL1. Here, the entire
compressive stress stored in the first region RG1 may be represented by
the area under a specific function in the depth direction of layers from
the first surface S1.

[0076] The first region RG1 can offset a tensile stress generated in the
glass substrate while the glass substrate is being folded, particularly,
a tensile stress reaching a maximum value in the vicinity of the first
surface S1.

[0077] In the second region RG2, the second depth DOL2 is a depth at which
a positive ion inside the glass substrate is exchanged with a positive
ion from the outside. Accordingly, the second region RG2 has a second
thickness T2. A second compressive stress CS2 is applied to the second
region RG2 by the ion exchange. The second compressive stress CS2
decreases from the second surface S2 to the second depth DOL2 according
to a predetermined function, and becomes 0 at the second depth DOL2. The
entire compressive stress stored in the second region RG2 may also be
represented by the area under a specific function in the depth direction
of layers from the second surface S2.

[0078] The second region RG2 can offset a tensile stress generated in the
glass substrate GL while the glass substrate GL is being folded,
particularly, a tensile stress reaching a maximum value in the vicinity
of the second surface S2.

[0079] The functions related to the above-described compressive stresses
may be changed depending on the type of the glass substrate, the type of
the ion-exchanged ion, condition(s) in the ion exchange, or the like, but
may decrease in the direction from the first surface S1 to the third
region RG3 and in the direction from the second surface S2 to the third
region RG3. In this illustrated embodiment, the two functions have been
shown in the form of straight lines for convenience of illustration.
However, the increase or decrease of the functions is not limited
thereto, and the function value to the first depth DOL1 or to the second
depth DOL2 may be variously changed. For example, the function value may
be constant, may decrease and then increase, or may increase and then
decrease.

[0080] In one or more exemplary embodiments, the first depth DOL1 and/or
the second depth DOL2 may be different depending on the thickness of the
glass substrate. For example, the first depth DOL1 and/or the second
depth DOL2 may be provided to be equal to or less than 1/3 or 1/4 of the
thickness of the glass substrate.

[0081] If it is assumed that the positive ion from the outside is a first
ion and the positive ion inside the glass substrate is a second ion, the
first ion may be K.sup.+ and the second ion may be Na.sup.+ or Li.sup.+.

[0082] The first compressive stress CS1 and the second compressive stress
CS2 are balanced by a central tension CT stored in the third region RG3.
The central tension CT is a tensile stress.

[0083] In one or more exemplary embodiments, the first depth DOL1 is the
substantially same as the second depth DOL2. In addition, the first
compressive stress CS1 at the first surface S1 of the glass substrate is
the substantially same as the second compressive stress CS2 at the second
surface S2 of the glass substrate. As a result, the compressive stresses
at both the surfaces of the glass substrate are symmetrical to each
other.

[0084] When the glass substrate is folded or rolled, a compressive stress
is applied to the surface corresponding to an inner circumferential
surface, and a tensile stress is applied to the surface corresponding to
an outer circumferential surface.

[0085] In one or more exemplary embodiments, in the glass substrate, each
of the first depth DOL1 and the second depth DOL2 may be about 1 .mu.m to
about 15 .mu.m. In another embodiment of the glass substrate, each of the
first depth DOL1 and the second depth DOL2 may be about 6 .mu.m to about
10 .mu.m.

[0086] Also, each of the first compressive stress CS1 and the second
compressive stress CS2 in the glass substrate, applied by the chemical
reinforcement, may be from about 600 MPa to about 1200 MPa. The chemical
reinforcement may be achieved by the ion exchange process. In another
embodiment, each of the first compressive stress CS1 and the second
compressive stress CS2, applied by the chemical reinforcement, may be
about 730 MPa to about 850 MPa. Here, the first compressive stress CS1
may be the entire compressive stress stored in the first region RG1, and
the second compressive stress CS2 may be the entire compressive stress
stored in the second region RG2.

[0087] When a flaw is generated in at least one of the opposed surfaces of
the glass substrate, if the flaw exists within a region defined by a
reference line where the compressive stress is 0 and a positive
compressive stress value, the tensile strength is compensated by the
compressive strength. In this case, the glass substrate GL is not broken,
and the damage caused by the flaw is prevented. When the first depth DOL1
and the second depth DOL2 are out of the above-described range, that is,
when the flaw does not exist within the above-described region and/or
when the first compressive stress CS1 and the second compressive stress
CS2 are out of the above-described range, it is difficult to sufficiently
compensate for the tensile stress applied to the glass substrate.

[0088] In one or more exemplary embodiments, the chemical reinforcement
may be symmetrically performed on the two opposed surfaces of the glass
substrate. However, the inventive concepts are not limited thereto, and
the chemical reinforcement may be asymmetrically performed. Particularly,
when the glass substrate is mainly folded in a specific direction, the
chemical reinforcement for compensating for the stresses may also be
asymmetrically performed.

[0089] In another embodiment, chemical reinforcement may be performed on
each of the two glass substrates constituting the window substrate WD to
the same degree or different degrees. For example, the first glass
substrate GL1 and the second glass substrate GL2 may be chemically
reinforced to have compressive stresses equal to each other. However, the
inventive concepts are not limited thereto, and the first glass substrate
GL1 and the second glass substrate GL2 may be chemically reinforced to
have compressive stresses different from each other. In one or more
exemplary embodiments the second glass substrate GL2 may be chemically
reinforced to have a compressive stress greater than that of the first
glass substrate GL1. Therefore, the second glass substrate GL2 may have a
greater depth than the first glass substrate GL1.

[0090] As described above, the damage of the glass substrate, i.e., the
first glass substrate GL1 and the second glass substrate GL2 is minimized
even in a case where a flaw is generated in the surface to which the
tensile stress is applied when the glass substrate is folded or rolled.
Particularly, although the first and second glass substrates GL1 and GL2
are ultra-thin substrates having a thickness of 50 .mu.m or less, the
generation or growth of a flaw due to the tensile stress when the first
and second glass substrates GL1 and GL2 are folded or rolled can be
decreased. Accordingly, the possibility of damage of the entire window
substrate can be surprisingly decreased.

[0091] The light transmittance of the window substrate WD having the
above-described structure may be about 90% or more. Light emitted from
pixels of a display panel which will be described later may be viewed by
a user through the window substrate WD. Also, the light emitted from the
pixels may pass through the window substrate WD. The window substrate has
a sufficient light transmittance, so that the luminance of light emitted
from the display panel can be prevent from being lowered.

[0092] In one or more exemplary embodiments, the window substrate WD may
have a yellow index variation (.DELTA.Y1) of 2.0 or less even when the
window substrate is exposed to UVB light having a wavelength of 280 nm to
360 nm for 72 hours. When the window substrate WD is applied to another
device such as a display device, the window substrate WD may be exposed
to direct light including UV. However, any color variation does not occur
in the window substrate WD under such a condition. The window substrate
WD constructed according to the principles of the invention satisfies
this condition.

[0093] In one or more exemplary embodiments, the window substrate WD may
have a haze value of 1.5% or less. In one or more exemplary embodiments
the window substrate may have a pencil hardness of 8 H to 9 H.

[0094] In one or more exemplary embodiments, as the window substrate WD
includes the first glass substrate GL1, the second glass substrate GL2,
and the interlayer ML provided between the first and second glass
substrates GL1 and GL2, the window substrate WD can have an impact
resistance. Particularly, bending deformation due to a point impact and
compressive deformation and/or tensile deformation due to a surface
impact are reduced.

[0095] The bending deformation due to the point impact may be checked
through a pen drop impact resistance test. In one or more exemplary
embodiments, an impact resistance against a point impact was evaluated in
such a manner that checks whether the window substrate is damaged by
allowing a specific pen (Fine BIC pen produced by Societe BIC) having a
weight of about 5.8 g, which is covered with a lid, to freely drop,
according to gravity, in a state in which the specific pen is
perpendicular to a surface of the window substrate. In one or more
exemplary embodiments, the window substrate may have an impact resistance
of at least about 4 cm. That is, when the pen having the weight of about
5.8g drops toward the window substrate at a height of about 4 cm or less,
the window substrate may not be broken.

[0096] One surface of the window substrate is frequently in contact with a
sharp tool such as a stylus pen. The impact of a front surface, caused by
the tool, has a relatively narrow contact area with the window substrate,
and accordingly corresponds to a point impact where a high pressure is
applied to a narrow area of the window substrate when the tool drops from
the top. As a result, bending deformation occurs as the window substrate
is bent at the position at which the impact is applied. Therefore, the
bending deformation causes damage of the window substrate or causes a
bright spot failure of the display panel. However, a window substrate
constructed according to the principles of the invention has a sufficient
impact resistance against the point impact, and thus the above-described
damage or failure can be prevented.

[0097] The compressive deformation and/or tensile deformation due to the
surface impact may be checked through an impact resistance test.

[0098] In general, a stress applied to a glass substrate may be indicated
by application load (N)/unit area (m.sup.2). When a tool such as a ball
applies an impact to the glass substrate, the durability against impact
is improved as the area in which the ball is applied to the glass
substrate increases when the ball is in contact with the glass substrate.
In an embodiment, first and second glass substrates are provided, and an
interlayer is provided between the first and second glass substrates, so
that, although a tool such as a ball applies an impact to one of the two
substrates, the volume at a point at which an accumulated stress forms
and grows a defect such as a crack increases, thereby improving impact
resistance (Fracture criterion for glass under impact loading, S. Bouzid
et al., International Journal of Impact Engineering 25 (2001) 831-845).

[0099] In one or more exemplary embodiments, the impact resistance against
surface impact was evaluated in such a manner that measures how much
impact load the window substrate absorbs by allowing an iron ball having
a diameter of 25. 4 mm and a mass of 5.5 g to freely drop under gravity
at a height of 5 cm from a surface of the window substrate. The window
substrate according to the embodiment can absorb at least about 5% of the
impact load.

[0100] The window substrate is frequently in contact with a tool having a
relatively wide contact area, in addition to the sharp tool such as the
stylus pen. As the window substrate is compressed at an impact position
by a surface impact applied by the tool and stretched in the vicinity of
the impact position, compressive deformation and tensile deformation
occur. The compressive deformation and tensile deformation may also cause
damage of the window substrate and a bright spot failure of the display
panel. However, a window substrate constructed according to the
principles of the invention has a sufficient impact resistance against
the surface impact, and thus the above-described damage or failure can be
prevented.

[0101] In one or more exemplary embodiments, the window substrate may have
a folding reliability of 200,000 times. The folding reliability means
that the window substrate is not damaged even when it is folded multiple
times. A folding reliability may be performed in a plurality of cycles,
using a Clamshell folding reliability test. The Clamshell folding
reliability test may be performed in a form in which a step of allowing
both end portions of the window substrate to face each other and then
stretching the window substrate to be in a flat state is used as a cycle,
thereby repeating the cycle. In an embodiment, when the window substrate
is applied to another device, e.g., a display device, the window
substrate may be folded multiple times. Therefore, the folding
reliability test is performed as a test for ensuring reliability when the
window substrate is folded multiple times. In the folding reliability
test, the window substrate was attached to an optically clear adhesive
and then placed on a test plate. The test plate includes a stationary
plate and a rotatable plate rotatable between the stationary plate and a
single- or multi-shaft folding gear. In the folding reliability test, the
glass substrate is maintained flat for a predetermined time (e.g., about
one second) by disposing the window substrate on the stationary plate and
the rotatable plate, which are placed on the same plane, and the
rotatable plate is moved to be parallel to the stationary plate in a
state in which the rotatable plate is spaced apart from the stationary
plate. Accordingly, the window substrate is in a state in which it is
folded between the two plates while having a predetermined curvature. In
this state, the glass substrate is again maintained for the predetermined
time (e.g., about one second). Next, as the rotatable plate is returned
to the original state, the glass substrate remains flat in the initial
state by disposing the window substrate on the stationary plate and the
rotatable plate, which are placed on the same plane, so that one cycle is
completed. The folding velocity is 2.4 seconds/time, and the total time
required is 124 hours. The number of times of folding (cycle) is 200,000
times. When no abnormality occurs in the window substrate after the
window substrate is folded 200,000 times, the folding reliability of the
window substrate is evaluated as OK. When abnormality such as damage
occurs in the window substrate after the window substrate is folded
200,000 times, the folding reliability of the window substrate is
evaluated as NG.

[0102] Conditions in the folding reliability test may be changed. When the
rotatable plate is parallel to the stationary plate in the state in which
the rotatable plate is spaced apart from the stationary plate, the
distance between the rotatable plate and the stationary plate is D, and
D/2 substantially corresponds to the radius of curvature. The distance D
may be controlled to be about 1 mm to about 10 mm, using the folding
gear.

[0103] For example, the conditions in the folding reliability test may be
changed depending on a thickness of the window substrate WD, a target
radius of curvature, and a compressive stress after chemical
reinforcement by the ion exchange process. In addition, the folding
reliability test may be performed in one direction. However, the folding
reliability test may be performed in one direction and the opposite
direction thereof.

[0104] In one or more exemplary embodiments, the window substrate may have
a repulsive force of 20 N or less when the window substrate is deformed.
When the window substrate is applied to a display device, etc., the
window substrate may be folded plural times, and the repulsive force when
a user folds the window substrate WD is preferably to a degree such that
the user does not feel inconvenient.

[0105] The repulsive force against deformation of the window substrate may
be defined by the following Equations 1 and 2.

[0106] In Equations 1 and 2, E is Young's modulus, t is glass thickness, w
is width of the window substrate, and D is distance between both end
portions of the window substrate, which face each other in folding. The
folding is performed in a form in which the window substrate having the
width w is folded such that both the end portions of the window substrate
face each other. Since D corresponds to two times of the radius of
curvature of the window substrate, the window substrate may have a radius
of curvature of about 1 mm to about 10 mm or about 1 mm to about 5 mm,
which satisfies a value of D corresponding thereto.

[0107] In one or more exemplary embodiments, the window substrate has a
high bending strength. The bending strength may be checked through a 2
point bending test. The 2 point bending test is performed in such a
manner that prepares lower and upper jigs parallel to each other, places
a sample (e.g., 7.2 inches) having a predetermined size between the lower
and upper jigs in a state in which the sample is bent, and then applies a
load to the upper jig in a lower direction. At this time, the sample is
bent such that both end portions of the sample face each other. One of
both the end portions is in contact with the upper jig, and the other of
both the end portions is in contact with the lower jig. In the 2 point
bending test, the maximum load where the sample is not damaged is
measured. In an embodiment, the window substrate may exhibit a bending
strength of 1.3 GPa or more.

[0108] In one or more exemplary embodiments, the window substrate may have
a radius of curvature of about 1 mm to about 10 mm or about 1 mm to about
5 mm. In this case, there is no damage of the window substrate. In one or
more exemplary embodiments, although the window substrate is damaged,
shattering in the damage is prevented.

[0109] In one or more exemplary embodiments, it has been illustrated that
the window substrate includes the first glass substrate GL1, the second
glass substrate GL2, and the interlayer ML between the first and second
glass substrates GL1 and GL2. However, this is provided for convenience
of description, and the inventive concepts are not limited thereto. For
example, the window substrate may further include three or more glass
substrates, and interlayers may be provided between the respective glass
substrates.

[0110] FIG. 3 is a cross-sectional view of a second embodiment of a window
substrate constructed according to the principles of the invention.

[0111] Referring to FIG. 3, the window substrate WD may include a first
glass substrate GL1, a second glass substrate GL2, and a third glass
substrate GL3, which are sequentially stacked. A first interlayer ML1 may
be provided between the first glass substrate GL1 and the second glass
substrate GL2, and a second interlayer ML2 may be provided between the
second glass substrate GL2 and the third glass substrate GL3.

[0112] In this embodiment, thicknesses of each of the first to third glass
substrates GL1, GL2, and GL3 may be equal to one another, but in other
embodiment they may vary from one another. As described above, when the
window substrate WD is folded such that a portion of any one of the
surfaces of the window substrate WD and the rest of the surface face each
other, the thicknesses of the first to third glass substrates GL1, GL2,
and GL3 may be determined so as to minimize stresses applied to the first
to third glass substrates GL1, GL2, and GL3.

[0113] For example, the window substrate WD may be bent about a folding
line at a surface of the third glass substrate GL3, such that a portion
of the surface of the third glass substrate GL3 and the rest of the
surface of the third glass substrate GL3 face each other. In this case,
one surface of the first glass substrate GL1 becomes an outer
circumferential surface, and one surface of the third glass substrate GL3
becomes an inner circumferential surface. In this case, the thickness of
the first glass substrate GL1 may have a smaller value than those of the
second and third glass substrates GL2 and GL3. In the same manner, the
thickness of the second glass substrate GL2 may have a smaller value than
that of the third glass substrate GL3. In an embodiment, when the window
substrate WD is bent, thicknesses of the first interlayer ML1 and the
second interlayer ML2 may also have different values so as to enhance the
damping effect of the first interlayer ML1 and the second interlayer ML2.

[0114] FIG. 4 is a cross-sectional view of a third embodiment of a window
substrate constructed according to the principles of the invention.

[0115] Referring to FIG. 4, the window substrate WD may include various
functional layers. For example, the window substrate WD may further
include a cover layer CVL provided as a functional layer on one surface
thereof.

[0116] In one or more exemplary embodiments, the cover layer CVL may be
provided on a surface of the window substrate WD at a side facing a user.
That is, the cover layer CVL is a surface that is directly exposed to the
user, and may be provided on a surface with which a user's finger, a
stylus pen, or another external object is in contact.

[0117] The cover layer CVL may be an anti-reflection layer that minimizes
reflection on a surface of a glass substrate. The cover layer CVL may be
an anti-stain layer that prevents a stain or smudge such as a user's
handprint (e.g., a fingerprint), but inventive concepts are not limited
thereto. In addition, although the cover layer CVL is illustrated as a
single layer in FIG. 4, the inventive concepts are not limited thereto,
and a layer having various functions may be provided as a plurality of
layers.

[0118] In this embodiment, only the cover layer CVL disposed on the
surface facing the user in the window substrate has been illustrated as a
functional layer, but inventive concepts are not limited thereto. The
functional layer may be disposed on a back surface that does not face the
user. In this case, the functional layer may be an additional layer for
improving the impact resistance of the window substrate and preventing
scattering damage of the window substrate. The additional layer may
include at least one selected from urethane-based resin, epoxy-based
resin, polyester-based resin, polyether-based resin, acrylate-based
resin, acrylonitrile-butadiene-styrene (ABS) resin, and rubber. The
additional layer may be directly coupled to the window substrate. The
additional layer may be formed on the window substrate using a coating
technique. For example, the additional layer may be formed on the window
substrate using slip coating, bar coating, spin coating, or the like.

[0119] Although not shown in this figure, the window substrate WD may have
a first glass substrate GL1 and/or a second glass substrate GL2 formed in
various shapes, e.g., formed with a protruding part or recessed part.
According to this type of embodiment, the first glass substrate GL1
and/or the second glass substrate GL2 may have a recessed part that is
concave inwardly from a surface at the back surface. The shape of the
recessed part may be variously provided. When viewed in plan, the
recessed part may be formed with various sizes at various positions on
the glass substrate. For example, in consideration of the direction in
which the glass substrate is to be folded or rolled, the recessed part
may be provided in an area corresponding to the direction. For example,
when the glass substrate is folded or rolled such that one surface of the
glass substrate becomes an inner circumferential surface, the recessed
part may be provided on the one surface. Alternatively, the recessed part
may be formed in an area corresponding to the almost entire surface of
the glass substrate.

[0120] When the recessed part is provided in the entire area or a partial
area on the glass substrate, the thickness of the glass substrate in the
area in which the recessed part is provided decreases, and hence the
tensile stress generated in the glass substrate decreases. As a result,
the glass substrate is more easily bent or rolled.

[0121] The above-described window substrate WD may be incorporated with
various devices as recognized by the skilled artisan. Particularly, the
window substrate WD may be used as a protective substrate for protecting
a display panel to a display device.

[0122] FIG. 5 is a perspective view of a first embodiment of a display
device constructed according to the principles of the invention having a
window substrate according to FIG. 1A. FIG. 6A is a cross-sectional view
of the display device of FIG. 5 in an unfolded position taken along line
II-II' of FIG. 5. FIG. 6B is a cross-sectional view of the display device
of FIG. 5 in a folded position taken along line II-II'' of FIG. 5.

[0123] Referring to FIGS. 5, 6A, and 6B, the display device according to
this embodiment can be folded or curved.

[0124] The display device includes a display panel DP that displays an
image and a window substrate WD provided on one surface of the display
panel DP.

[0125] The display panel DP displays, on a front surface thereof,
arbitrary visual information, e.g., a text, a video, a picture, a
two-dimensional or three-dimensional image, etc. The kind of the display
panel DP is not limited as long as the display panel DP is capable of
displaying images.

[0126] The entire or at least a portion of the display panel DP may have
flexibility. For example, the display panel DP may have flexibility in
the entire area thereof. Alternatively, the display panel DP may have
flexibility in an area corresponding to a flexible area FA.

[0127] In the illustrated embodiment, the display panel DP is provided in
the shape of a plate having a front surface on which an image is
displayed and a back surface opposite to the front surface. The display
panel DP displays an image on the front surface. In one or more exemplary
embodiments, the display panel DP may display an image on both of the
front and back surfaces. However, in this embodiment, a case where an
image is displayed on the front surface is described as an example. In
FIG. 5, the direction of the front surface on which an image is displayed
has been indicated by an arrow.

[0128] The display panel DP includes a display area DA in which the image
is displayed and a non-display area NDA located at at least one side of
the display area DA. For example, the non-display area NDA may be
provided in a shape surrounding the display area DA.

[0129] The display area DA is an area in which a plurality of pixels PXL
is provided to display an image as is known in the art.

[0130] The display area DA may be provided in a shape corresponding to
that of the display device. For example, like the shape of the display
device, the display area DA may be provided in various shapes such as a
closed-shape polygon including linear sides, a circle, an ellipse, etc.,
including curved sides, and a semicircle, a semi-ellipse, etc., including
linear and curved sides. In the illustrated embodiment, the display area
DA is provided in a rectangular shape.

[0131] The pixels PXL are provided on the display area DA. Each of the
pixels PXL is a minimum unit for displaying an image, and may be provided
in plurality. The pixels PXL may emit white light and/or color light.
Each pixel PXL may emit light of one of red, green, and blue, but the
inventive concepts are not limited thereto. For example, each pixel PXL
may emit light of a color such as cyan, magenta, or yellow.

[0132] The pixel PXL may be a light emitting device including an organic
light emitting layer, but the inventive concepts are not limited thereto.
For example, the pixel may be implemented in various forms such as a
liquid crystal device, an electrophoretic device, and an electrowetting
device.

[0133] The pixels PXL will be described subsequently.

[0134] The window substrate WD is provided on the front surface of the
display panel DP, and includes a first glass substrate GL1, a second
glass substrate GL2, and an interlayer ML between the first glass
substrate GL1 and the second glass substrate GL2. The above-described
window substrates may be used as the window substrate WD, and therefore,
only differences with the above disclosure will be primarily described to
avoid redundancy.

[0135] The display panel DP may be provided on the first glass substrate
GL1 or the second glass substrate GL2 of the window substrate WD. In the
illustrated embodiment, the display panel DP is provided on the second
glass substrate GL2, so that the window substrate WD and the display
panel DP are arranged in a form in which the outer surface of the second
glass substrate GL2 and the front surface of the display panel DP are in
contact with each other. However, the arrangement of the window substrate
WD and the display panel DP is not limited thereto, and the display panel
DP may be provided on the first glass substrate GL1, so that the window
substrate WD and the display panel DP are arranged in a form in which the
outer surface of the first glass substrate GL1 and the front surface of
the display panel DP are in contact with each other.

[0136] Although not shown in these figures, an optically clear adhesive
layer may be provided between the display panel DP and the window
substrate WD. The optically clear adhesive layer between the display
panel DP and the window substrate WD may be selected from materials
constituting the interlayer ML. However, the material of the optically
clear adhesive layer is not limited thereto.

[0137] In this embodiment, the window substrate WD allows an image from
the display panel DP to be transmitted therethrough and simultaneously
reduces impact from the outside, so that it is possible to prevent the
display panel DP from being damaged or erroneously operated due to the
impact from the outside. The impact from the outside refers to a force
from the outside, such as stress, which causes a defect of the display
panel DP.

[0138] The window substrate WD, as described above, may have elasticity to
absorb the impact from the outside and simultaneously distribute the
impact from the outside to surroundings thereof. For example, the window
substrate WD is deformed by impact from the outside, and may have
elasticity that enables the window substrate WD to return to the original
state when the impact from the outside is eliminated. In addition, the
window substrate WD has an excellent impact resistance in which the
window substrate WD is less deformed or damaged due to impact from the
outside.

[0139] The window substrate WD reduces bending deformation of the display
panel DP due to a point impact and a surface impact and compressive
deformation and/or tensile deformation of the display panel DP due to a
surface impact, thereby preventing a failure of the display panel DP.

[0140] The entire or at least a portion of the window substrate WD may
have flexibility. For example, the display panel DP may have flexibility
in the entire area or a partial area thereof.

[0141] The display device including the display panel DP and the window
substrate WD may be provided in various shapes. For example, the window
substrate WD may be provided in plate shape corresponding to the shape of
the display panel DP, and may cover at least a portion of the front
surface of the display panel DP. For example, when the display panel DP
is provided in a rectangular shape, the window substrate WD may also be
provided in a rectangular shape corresponding to that of the display
panel DP. Alternatively, when the display panel DP is provided in a
circular shape, the window substrate WD may also be provided in a
circular shape corresponding to that of the display panel DP.

[0142] In one or more exemplary embodiments, although shapes of the
display panel DP and the window substrate WD have minute differences, the
display panel DP and the window substrate WD may be provided in
substantially the same shape. Accordingly, the display device including
the display panel DP and the window substrate WD may be provided in the
shape of a rectangular plate. For ease of description, the illustrated
embodiment shows a case where the display device is provided in a
rectangular shape having a pair of long sides and a pair of short sides.
In addition, FIG. 5 illustrates the longer sides extending in a first
direction D1 and the short sides extending in a second direction D2, and
the direction perpendicular to the extending directions of the long and
short sides is a third direction D3. However, the shape of the display
device is not limited thereto, and the display device may have various
shapes. For example, the display device may be provided in various shapes
such as a closed-shape polygon including linear sides, a circle, an
ellipse, etc., including curved sides, and a semicircle, a semi-ellipse,
etc., including linear and curved sides. In an embodiment, when the
display device has linear sides, at least one portion of corners of each
of the shapes may be formed in a curve. For example, when the display
device has a rectangular shape, a portion at which adjacent linear sides
meet each other may be replaced with a curve having a predetermined
curvature. That is, a vertex portion of the rectangular shape may be
formed with a curved side having both adjacent ends respectively
connected to two adjacent linear sides, with the curved side having a
predetermined curvature. The curvature may be differently determined
depending on position. For example, the curvature may be changed
depending on the position at which the curve is started, a length of the
curve, etc.

[0143] At least one portion of the display device may have flexibility,
and the display device may be folded about a folding line or axis
provided in the portion having the flexibility. That is, the display
device may include a flexible area FA having the flexibility, the
flexible area FA being the area in which the display device is foldable,
and a rigid area RA provided at at least one side of the flexible area,
the rigid area RA being the area(s) in which the display device is not
folded.

[0144] In this embodiment, an area in which the display panel is not
folded is referred to as the rigid area RA, but this is provided for
convenience of description. The term "rigid" includes not only the case
where the rigid area is hard without any flexibility but also the case
where the rigid area has a smaller amount of flexibility than the
flexible area FA and a case where the rigid area has flexibility but is
not foldable.

[0145] In one or more exemplary embodiments, the entire display device may
correspond to the flexible area FA. For example, in the case of a display
device bent into a roll shape, the entire display device may correspond
to the flexible area.

[0146] FIG. 5 illustrates an exemplary embodiment in which a first rigid
area RA1, the flexible area FA, a second rigid area RA2 are sequentially
arranged along the first direction D1. The flexible area FA extends
longitudinally across the display area DA along the second direction D2.

[0147] When assuming that the center line about which the display device
is folded is a folding line FL, the folding line FL is provided in the
flexible area FA. This embodiment illustrates that the folding line FL
passes through the center of the flexible area FA, and the flexible area
FA is symmetrically disposed with respect to the folding line FL, but the
inventive concepts are not limited thereto. That is, the folding line FL
may be provided asymmetrically in the flexible area FA. The flexible area
FA and the folding line FL on the flexible area FA may overlap with an
area in which an image of a display panel DP is displayed. When the
display device is folded, the area in which the image is displayed may be
bent.

[0148] This embodiment illustrates a state in which one surface of each of
the two rigid areas are located in parallel to each other and folded to
face each other. However, the inventive concepts are not limited thereto.
For example, the surfaces of the two rigid areas may be folded to form a
predetermined angle (e.g., an acute angle, a right angle, or an obtuse
angle) with the flexible area interposed therebetween.

[0149] In the display device, the folding line FL is provided in the
flexible area FA along the second direction D2 that is the extending
direction of the flexible area FA. Accordingly, the display device can be
folded in the flexible area FA.

[0150] In one or more exemplary embodiments, when the display device is
folded along the folding line FL, the display device may be folded such
that two portions of a surface opposite to a surface on which an image is
displayed face each other. A window substrate WD may be exposed at the
outermost side in the state in which the display device is folded. In
this case, the image can be provided to the outside even in the state in
which the display device is folded, and a user can view the image
regardless of whether the display device is folded. However, the folding
direction of the display device is not limited thereto. When the display
device is folded along the folding line FL, unlike as shown in FIG. 6B,
the display device may be folded such that two portions of the surface on
which the image is displayed face each other.

[0151] For convenience of description, this embodiment illustrates that
the first and second rigid areas RA1 and RA2 have areas similar to each
other, and the flexible area FA is located between the two rigid areas
RA1 and RA2, but the inventive concepts are not limited thereto. For
example, the first and second rigid areas RA1 and RA2 may have areas
different from each other. In addition, the number of rigid areas is not
necessarily two, and may be one or three or more. In this case, a
plurality of rigid areas may be provided to be spaced apart from each
other with the flexible area FA interposed therebetween.

[0152] In one or more exemplary embodiments, the flexible area FA and the
rigid areas of the window substrate WD may have different thicknesses,
different repulsive forces, different impact resistances, and the like.
When repetitive bending occurs in a specific area (e.g., the flexible
area FA) even though the window substrate WD is entirely flexible, stress
in the specific area may be increased as compared with the other areas,
and the window substrate WD may be deformed such that damage from bending
stresses is prevented.

[0153] For example, each index of the window substrate WD in the flexible
area FA may be differently determined by changing the thickness of the
first glass substrate GL1 and/or the second glass substrate GL2 or by
changing the depth where chemical reinforcement is performed on the first
glass substrate GL1 and/or the second glass substrate GL2.

[0154] Next, each pixel in the display panel will be described.

[0155] FIG. 7 is a block diagram of the major electronic components of a
display panel DP constructed according to the principles of the
invention. FIG. 8 is a circuit diagram of one of the pixels PXL in FIG.
7.

[0156] Referring to FIGS. 7 and 8, the display panel DP constructed
according to the principles of the invention includes pixels PXL provided
on a display area DA, gate and data drivers GDV and DDV for driving the
pixels PXL, and a timing controller TCN for controlling the driving of
the gate and data drivers GDV and DDV.

[0157] Each pixel PXL is provided on the display area DA, and includes a
line unit including a gate line GL, a data line DL, and a driving voltage
line DVL, a thin film transistor connected to the line unit, an organic
light emitting device EL connected to the thin film transistor, and a
capacitor Cst.

[0158] The gate line GL extends in one direction. The data line DL extends
in another direction intersecting the gate line GL. The driving voltage
line DVL extends in the substantially same direction as the data line DL.
The gate line GL transmits a gate signal to the thin film transistor. The
data line DL transmits a data signal to the thin film transistor. The
driving voltage line DVL provides a driving voltage to the thin film
transistor.

[0159] The thin film transistor may include a driving thin film transistor
TR2 for controlling the organic light emitting device EL and a switching
thin film transistor TR1 for switching the driving thin film transistor
TR2. In the illustrated embodiment one pixel PXL includes two thin film
transistors TR1 and TR2. However, the inventive concepts are not limited
thereto, and the one pixel PXL may include one thin film transistor and
one capacitor, or may include three or more thin film transistors and two
or more capacitors.

[0160] A gate electrode of the switching thin film transistor TR1 is
connected to the gate line GL, and a source electrode of the switching
thin film transistor TR1 is connected to the data line DL. A drain
electrode of the switching thin film transistor TR1 is connected to a
gate electrode of the driving thin film transistor TR2. The switching
thin film transistor TR1 transmits, to the driving thin film transistor
TR2, a data signal applied to the data line DL, in response to a gate
signal applied to the gate line GL.

[0161] The gate electrode of the driving thin film transistor TR2 is
connected to the drain electrode of the switching thin film transistor
TR1, and a source electrode of the driving thin film transistor TR2 is
connected to the driving voltage line DVL. A drain electrode of the
driving thin film transistor TR2 is connected to the organic light
emitting device EL.

[0162] The organic light emitting device EL includes a light emitting
layer (not shown), and a first electrode (not shown) and a second
electrode (not shown), which are opposite to each other with the light
emitting layer interposed therebetween. The first electrode is connected
to the drain electrode of the driving thin film transistor TR2. A common
voltage is applied to the second electrode, and the light emitting layer
emits light according to an output signal of the driving thin film
transistor TR2, so that light is emitted or is not emitted, thereby
displaying an image. The light emitted from the light emitting layer may
be white light or color light.

[0163] The capacitor Cst may be connected between the gate electrode and
the source electrode of the driving thin film transistor TR2. The
capacitor Cst charges and maintains a data signal input to the gate
electrode of the driving thin film transistor TR2.

[0164] The timing controller TCN receives a plurality of image signals RGB
and a plurality of control signals CS from an outside of the display
device. The timing controller TCN converts data formats of the image
signals RGB to be suitable for interface specifications with the data
driver DDV, and provides the converted image signals R'G'B' to the data
driver DDV. In addition, the timing controller TCN generates a data
control signal D-CS (e.g., an output start signal, a horizontal start
signal, etc.) and a gate control signal G-CS (e.g., a vertical start
signal, a vertical clock signal, vertical clock bar signal, etc.), based
on the plurality of control signals CS. The data control signal D-CS is
provided to the data driver DDV, and the gate control signal G-CS is
provided to the gate driver GDV.

[0165] The gate driver GDV sequentially output a gate signal in response
to the gate control signal G-CS provided from the timing controller TCN.
Thus, the plurality of pixels PXL can be sequentially scanned in unit of
rows by the gate signal.

[0166] The data driver DDV converts the image signals R'G'B' into data
signals and outputs the converted data signals, in response to the data
control signal D-CS provided from the timing controller TCN. The output
data signals are applied to the pixels PXL.

[0167] Thus, each pixel PXL is turned on by the gate signal, and the
turned-on pixel PXL receives a corresponding data voltage from the data
driver DDV, thereby displaying an image having a desired gray level.

[0168] FIG. 9 is a perspective view illustrating a display device
according to an embodiment, in which two display areas are provided.

[0169] Referring to FIG. 9, the display device may include a plurality of
display areas. For example, the display device may include a first
display area DA1 and a second display area DA2. A non-display area NDA
may be provided at the outer periphery of the first display area DA1 and
the second display area DA2, and the first display area DA1 and the
second display area DA2 may be spaced apart from each other with the
non-display area NDA interposed therebetween. When viewed in plan, a
display panel DP may include a first rigid area RA1, a second rigid area
RA2, and a flexible area FA disposed between the first rigid area RA1 and
the second rigid area RA2. The flexible area FA may overlap with the
non-display area NDA between the first display area DA1 and the second
display area DA2.

[0170] Like the above-described embodiments, a folding line FL may be
provided substantially parallel to any one side of the display device.
However, the inventive concepts are not limited thereto, and the folding
line FL may be disposed in various directions regardless of the shape of
the display device. For example, in one or more exemplary embodiments,
the folding line FL may be provided oblique to any one side of the
display device.

[0171] FIG. 10 is a plan view illustrating a display device according to
one or more embodiments, schematically illustrating only two rigid areas,
a flexible area, and a folding line.

[0172] Referring to FIG. 10, in this embodiment, the display device is
provided in a rectangular shape, and a folding line FL is provided along
a diagonal line of the rectangular shape. In the display device, a
flexible area FA is also provided in a diagonal direction along the
folding line FL, and a first rigid area RA1 and a second rigid area RA2
may be provided at both sides of the flexible area FA, respectively.

[0173] In one or more exemplary embodiments, a single folding line may be
provided. However, the inventive concepts are not limited thereto, and
plural folding lines FL may be provided.

[0174] FIG. 11A is a perspective view illustrating a third embodiment of a
display device constructed according to the principles of the invention
having a window substrate according to FIG. 1A foldable about two folding
lines. FIG. 11B is a perspective view illustrating the display device of
FIG. 11A in a folded position.

[0175] Referring to FIGS. 11A and 11B, a plurality of folding lines are
provided, and therefore, the display device may have a plurality of
flexible areas and a plurality of rigid areas. This embodiment describes
as an example two folding lines, i.e., a first folding line FL1 and a
second folding line FL2, and correspondingly, a first rigid area RA1, a
first flexible area FA1, a second rigid area RA2, a second flexible area
FA2, and a third rigid area RA3 sequentially disposed in a first
direction D1. The first flexible area FA1 and the second flexible area
FA2 may extend in a second direction D2 corresponding to the first
folding line FL1 and the second folding line FL2, respectively.

[0176] As shown in FIG. 11B, the display device may be folded in the first
flexible area FA1 and the second flexible area FA2. This figure
illustrates that the display device is folded such that the third rigid
area RA3 is located between the first rigid area RA1 and the second rigid
area RA2, but the inventive concepts are not limited thereto. In another
embodiment, the display device may be folded such that the second rigid
area RA2 is located between the first rigid area RA1 and the third rigid
area RA3.

[0177] This embodiment illustrates both of the first folding line FL1 and
the second folding line FL2 extending in the second direction D2.
However, the first folding line FL1 and the second folding line FL2 may
extend in different directions and directions different from each other.
For example, the first folding line FL1 may extend in the first direction
D1, and the second folding line FL2 may extend in the second direction
D2. Alternatively, both of the first folding line FL1 and the second
folding line FL2 extend in the first direction D1. Alternatively, the
first folding line FL1 may extend in the first direction D1, and the
second folding line FL2 may extend in a direction oblique to the first
folding line FL1. In another embodiment, three or more folding lines may
extend in any of the directions or arrangements noted above.

[0178] In one or more exemplary embodiments, a flexible area may be
provided at an outermost side of the display device in one direction such
that a portion of the display device is rolled like a roll.

[0179] FIG. 12A is a perspective view illustrating a fourth embodiment of
a display device constructed according to the principles of the invention
having a window substrate according to FIG. 1A foldable about a single,
offset folding line. FIG. 12B is a perspective view illustrating the
display device of FIG. 12A in a rolled position.

[0180] Referring to FIGS. 12A and 12B, the display device has a rigid area
RA and a flexible area FA. The flexible area FA may be provided at one
side of the rigid area RA. The flexible area FA may be provided at an
outermost side of the display device in a first direction D1. The display
device may be rolled in the flexible area FA.

[0181] The display device having the above-described structure includes
the above-described window substrate, so that the impact resistance of
the display device is improved. Hence, specific evaluation of certain
characteristics and test will be described in the following examples.

[0183] The following Table 1 shows results obtained by testing optical
characteristics and mechanical characteristics of existing window
substrates (Comparative Example 1 and Comparative Example 2) and a window
substrate constructed according to the principles of the invention
(Embodiment).

[0184] Each of Comparative Example 1, Comparative Example 2, and
Embodiment was fabricated as a substrate having an aspect ratio of 16:9,
the substrate of which diagonal line has a length of 7.2 inches (180
mm.times.105.2 mm).

[0185] Since Comparative Example 1 was fabricated as a polyimide
substrate, the substrate was formed in two layers each having a thickness
of 50 .mu.m, and the total thickness of the substrate was 100 .mu.m.
Comparative Example 2 was fabricated as a glass substrate having a
thickness of 50 .mu.m. Embodiment was fabricated to include a first glass
substrate having a thickness of 30 .mu.m, an interlayer having a
thickness of 10 .mu.m, and a second glass substrate having a thickness of
50 .mu.m.

[0186] In Table 1, the compressive stress refers to the compressive stress
of each glass substrate in Comparative Example 2 and Embodiment. The
chemical reinforcement depth refers to an ion exchange depth (depth of
length) of each glass substrate in Comparative Example 2 and Embodiment.
The bending strength refers to a maximum load checked through the 2 point
bending test. The impact absorption load refers to a load that each of
the window substrates of Comparative Example 1, Comparative Example 2,
and Embodiment absorbs when a load of 91.2 gf was applied to the window
substrate by an iron ball having a diameter of 25.4 mm and a mass of 5.5
g freely dropped at a height of 5 cm above a surface of the window
substrate. The folding reliability represents whether the example passed
a Clamshell folding reliability of 200,000 cycles. The thickness
homogeneity represents an average of differences in thickness at 9 points
specified at an equal distance in lateral and longitudinal directions
with respect to each window substrate.

[0187] Referring to Table 1, Embodiment exhibits optical characteristics
and mechanical characteristics, which correspond to those of Comparative
Example 1 and Comparative Example 2. Particularly, the impact resistance
(pen drop) of Embodiment is 4 cm, which is an improved value compared to
3 cm of Comparative Example 1 and 2 cm of Comparative Example 2. The
impact absorption load of Embodiment also exhibited a surprisingly larger
value than Comparative Example 1 and Comparative Example 2.

[0188] 2. Evaluation of Impact Resistances (Pen Drop)

[0189] The following Table 2 shows results obtained by testing impact
resistances (pen drop) of an existing window substrate (Comparative
Example) and window substrates constructed according to principles of the
invention (Embodiment 1 to Embodiment 5).

[0190] Referring to Table 2, in the Comparative Example configured with
only a glass substrate having a thickness of 50 .mu.m, damage to the
window substrate occurred at a height of 2 cm in the pen drop impact
resistance test. On the other hand, in Embodiment 1 to Embodiment 5,
damage to all of the window substrates occurred at a height of 4 cm or
more in the pen drop impact resistance test. Accordingly, it can be seen
that the impact resistances against point impact in Embodiment 1 to
Embodiment 5 are improved.

[0191] In addition, even when the sum of thicknesses of first and second
glass substrates is 100 .mu.m or less, damage to all of the window
substrates occurred at a height of 4 cm or more in the pen drop impact
resistance test.

[0192] 3. Evaluation of Impact Resistances (Ball Drop)

[0193] The following Table 3 shows results obtained by testing impact
resistances (ball drop) of existing window substrates (Comparative
Example 2 to Comparative Example 5) and a window substrate constructed
according to principles of the invention (Embodiment). FIG. 13 is a graph
illustrating impact resistances of an existing window substrate and a
window substrate obtained from Table 3.

[0194] The impact resistance against ball drop is evaluated in a manner
that measures how much impact load the window substrate absorbs when an
iron ball having a diameter of 25. 4 mm and a mass of 5.5 g is freely
dropped at a height of 5 cm above a surface of each window substrate.

[0195] Comparative Example 1 exhibits the impact load when the iron ball
is dropped without any window substrate, which impact load corresponds to
91.2 gf. Comparative Example 2 to Comparative Example 5 and Embodiment
were tested using a substrate having an aspect ratio of 16:9, the
substrate of which diagonal line has a length of 7.2 inches (180
mm.times.105.2 mm). Comparative Example 2 was fabricated as a window
substrate in which a hard coating layer having a thickness of 50 .mu.m
was formed on a polyimide film having a thickness of 50 .mu.m, and the
total thickness of the window substrate was provided as 100 .mu.m.
Comparative Example 3 was provided as Gorilla Glass 3, which is a
commercial product available from Corning, Inc., having a thickness of 50
.mu.m. Comparative Example 4 was provided as Gorilla Glass 3 having a
thickness of 80 .mu.m. Comparative Example 5 was fabricated to include a
polyimide film having a thickness of 40 .mu.m and Gorilla Glass 3 having
a thickness of 50 .mu.m. Embodiment was fabricated to include a first
glass substrate (Gorilla Glass 3) having a thickness of 30 .mu.m, an
interlayer (optically clear adhesive) having a thickness of 10 .mu.m, and
a second glass substrate (Gorilla Glass 3) having a thickness of 50
.mu.m.

[0196] Referring to Table 3, Comparative Example 2 can be bent with a
radius of curvature of 5 mm, but has a low absorption load. Comparative
Example 3 and Comparative Example 4 cannot be bent with the radius of
curvature of 5 mm. However, Comparative Example 5 and Embodiment can be
bent with the radius of curvature of 5 mm. In addition, Comparative
Example 5 and Embodiment have absorption loads of 5% or more.

[0197] 4. Evaluation of Repulsive Forces

[0198] The following Table 4 shows results obtained by evaluating
repulsive forces when the thicknesses of first and second glass
substrates are different from each other in window substrates constructed
according to principles of the invention.

[0199] Referring to Table 4, when the thickness of the first glass
substrate is within a range of 30 .mu.m to 50 .mu.m and the thickness of
the second glass substrate is within a range of 30 .mu.m to 50 .mu.m, all
of the embodiments exhibit repulsive forces of 20 N or less.

[0200] In addition, repulsive forces of Embodiment 2 and Embodiment 4 are
5.2 N and 6.5 N, respectively, and a repulsive force when the first glass
substrate is thinner than the second glass substrate has a smaller value
than that when the first glass substrate is thicker than the second glass
substrate. Accordingly, it can be seen that the thickness of a glass
substrate located at the outside is decreased according to a direction in
which the window substrate is curved, thereby decreasing the repulsive
force.

[0201] 4. Evaluation of Impact Resistance (Surface Impact)

[0202] The following Table 5 shows results obtained by evaluating impact
loads when the thicknesses of first and second glass substrates are
constant, but the thicknesses of interlayers are different in window
substrates constructed according to principles of the invention. FIG. 14
is a graph illustrating Table 5. That is, FIG. 14 is a graph illustrating
impact loads of in window substrates constructed according to the
principles of the invention when the thickness of first and second glass
substrates are constant but the thickness of the interlayers varies.

[0203] The impact loads were evaluated using Drop tower Impact System as
evaluation equipment, and evaluation was performed according to ASTM
D3763 evaluation standard. The evaluation was performed in a manner that
measures a load (N) at a failure point at which each window substrate is
damaged when a part of 1.954 kg is freely dropped at a height of 10 cm.
In this case, the impact speed in the evaluation corresponds to 0.77 m/s,
and the impact energy in the evaluation corresponds to 0.579 J.

[0204] Referring to Table 5, the impact load decreases as the thickness of
the interlayer increases. Hence, whenever the thickness of the interlayer
increases by about 10 .mu.m, the impact load decreases by about 5%.

[0205] 5. Evaluation of Maximum Stresses

[0206] The following Table 6 shows results obtained by measuring maximum
stress values in display devices employing existing window substrates
(Comparative Example 1 to Comparative Example 3) and window substrates
constructed according to principles of the invention (Embodiment 1 to
Embodiment 3). Table 6 shows result values obtained by measuring maximum
stresses applied to display panels, when the existing window substrates
and the window substrates embodiments were disposed on the respective
display panels, and an iron ball having a diameter of 25. 4 mm and a mass
of 5.5 g was freely dropped at a height of 5 cm above each of the window
substrates.

[0208] Referring to Table 6, Comparative Example 1 to Comparative Example
3 all exhibit maximum stresses of 18 MPa or more, but Embodiment 1 to
Embodiment 3 all exhibit maximum stresses of 17.3 MPa or less.
Particularly, it can be seen that, whenever the thickness of an
interlayer increases by about 10 .mu.m, the damping effect is improved by
about 4%.

[0209] 5. Evaluation of Folding Reliability

[0210] The following Table 7 shows results obtained by measuring folding
reliabilities in display devices employing window substrates constructed
according to principles of the invention (Embodiment 1 to Embodiment 7
and Comparative Example 1). In Table 7, 10 .mu.m to 100 .mu.m as the
thickness of each interlayer means that a test was performed by changing
the thickness in units of 10 .mu.m. That is, the folding reliability was
tested by changing the thickness of the interlayer to 10 .mu.m, 20 .mu.m,
30 .mu.m, . . . , 100 .mu.m.

[0211] Referring to Table 7, it can be seen that, when the thickness of
the interlayer is about 100 .mu.m or less, the folding reliability is
high. However, it can be seen that, when the total thickness of the
window substrate exceeds 190 .mu.m, the folding reliability was evaluated
as NG. The display devices according to inventive embodiments are not
limited to the above-described shapes, but may have various shapes.

[0212] The window substrate and the display device having the same
constructed according to principles of the invention may be employed in
various electronic devices. For example, the display device may be
applied to televisions, notebook computers, cellular phones, smart
phones, smart pads, PMPs, PDAs, navigations, various wearable devices
such as smart watches.

[0213] According to the inventive concepts, it is possible to provide a
window substrate is capable of ensuring durability and user's safety, and
a display device having the window substrate.

[0214] Although certain exemplary embodiments and implementations have
been described herein, other embodiments and modifications will be
apparent from this description. Accordingly, the inventive concepts are
not limited to such embodiments, but rather to the broader scope of the
presented claims and various obvious modifications and equivalent
arrangements.